1. Field of the Invention
The present invention relates to a transparent conductive coating (TCC) and more particularly to a TCC formed from bulk gallium nitride GaN on a sapphire substrate that is configured to compensate for lattice mismatches at its interfaces, for example, the interface between the GaN and a sapphire substrate and the interface between the GaN and a solar cell.
2. Description of the Prior Art
Various opto-electronic devices require transparent electrical conductors that are conductive in the frequency range from DC to radio frequency (RF) and transparent to visible light. Such transparent electrical conductors are known to be applied to such opto-electronic devices in the form of a coating, and have become known as transparent conductive coating (TCC) materials.
Various applications of such TCC materials are known. For example, such TCC materials are known to be used for electrically resistive heating systems for aircraft windshields, as well as in satellite applications. Solar cells are also known to use such TCC materials. In particular, in solar cells applications, the TCC material is used for conducting solar photon-generated currents from the surface of the solar cells, without causing the solar cell to be obscured. Such TCC materials are also known to be used in various other opto-electronic applications, such as liquid crystal displays, CCD camera sensors and photocopiers, as well as a myriad of other opto-electronic type devices.
Various semiconductor coatings with a relatively wide band gap are known to be used for such TCC materials. Specifically, materials having a band gap greater than the energy of the photons of light passed therethrough are known to be used. For transparency across the entire visible/near-infrared (VNIR) band, materials with band gaps wider than 3 eV are known to be used.
In many known applications of such TCC materials, electrical conductivity for such TCC materials approaching that of metals is required. In order for the material to be electrically conductive, one or more of the electron energy bands of the material must be partially filled. In relatively high conductive materials, a partially filled electron energy band normally dominates the conduction.
The density of carriers in the electron energy band, n, required for a specific conductivity, is given by Equation(1).
n=xcex4/qxcexc,xe2x80x83xe2x80x83(1)
where q is the electronic charge,
xcexc is the carrier mobility, and
xcex4 is the electrical conductivity.
To obtain a sufficient density of carriers in an electron energy band for the desired conductivity, the material is known to be doped because the Fermi level of the intrinsic (pure) material is normally deep within the band gap. However, doping is known to reduce the transmittance of the material for several reasons. First, the optical absorption of free carriers increases with the increasing concentration of carriers, as generally discussed in xe2x80x9cOptical Processes in Semiconductorsxe2x80x9d, by J. I. Pankove, Dover Publications, 1971, p.75. Second doping is known to change the density of states function, producing a tail on the absorption near the band edge, as generally discussed in xe2x80x9cAbsorption Edge of Impure Gallium Arsenidexe2x80x9d, by J. I. Pankove, Physical Review A, Vol. 140, 1965, pp. 2059-2065. The increase in absorption as a function of the doping level thus causes a fundamental trade-off in such TCC materials between electrical conductivity and VNIR transmittance.
Tin-doped indium oxide (ITO) is known to be used for such TCC material applications. As generally set forth in xe2x80x9cTransparent Conductorsxe2x80x94A Status Reviewxe2x80x9d, by K. L. Chopra, S. Major, and D. K. Pandya, Thin Film Solids, Vol. 102, 1983, pps. 1-46, such ITO coatings are known to have an electron mobility ranging from 15-40 cm2NV-s. In many known commercial and aerospace applications, transparent electrical conductors having a sheet electrical conductance of 1 or less ohms per square and a visible light transparency of 90% or better is required. A sheet electrical impedance of one ohm per square of the ITO coating requires a doping concentration of about 2xc3x971021 cmxe2x88x923. Unfortunately, such highly doped ITO coatings provide less than approximately 75% VNIR transmittance.
Such TCC""s are known to be used in solar cell applications, for example, gallium arsenide (GaAs) solar cells. Such GaAs solar cells are normally include a TCC formed on a germanium substrate and are covered with a glass cover. Because of its superior electron mobility, gallium nitride GaN is known to be used for such TCCs. Unfortunately, when used in a GaAs solar cell application, the GaN is lattice mismatched at the solar cell interface. Such lattice mismatches are a major concern for GaAs solar cells since such mismatches are known to generate dislocations or defects that act as traps which decrease the minority carrier diffusion length resulting in a loss in the overall efficiency of the solar cell.
Another problem with such GaAs solar cells relates to the cost. In particular, many known GaAs solar cells are known to be formed on germanium substrates. However, such germanium substrates are relatively expensive, resulting in a relatively high manufacturing cost of the solar cells. Another problem with GaAs solar cells is the need for a separate cover glass which further drives up the cost. Thus there is a need for TCCs that are adapted to be used with GaAs solar cells which account for the lattice mismatches at the interface of the TCC. There is also a need to eliminate the expensive germanium substrate and the separate cover glass from GaAs solar cells in order to reduce costs.
Briefly, the present invention relates to a transparent conductive coating (TCC) formed from GaN on a sapphire substrate. In order to account for the lattice mismatch between the GaN and the sapphire substrate, a gallium nitride nucleation layer is formed on the sapphire substrate. A mask, for example, silicon dioxide SiO2, is formed on top of the GaN nucleation layer with a plurality of openings. GaN is grown through the openings in the mask to form a lateral epitaxial overgrowth layer upon which defect-free GaN is grown. The lateral epitaxial overgrowth compensates for the lattice mismatch between the sapphire substrate and the GaN. The use of a sapphire substrate eliminates the need for a cover glass and also significantly reduces the cost of the TCC, since such sapphire substrates are about {fraction (1/7)} the cost of germanium substrates. The TCC may then be disposed on a GaAs solar cell. In order to compensate for the lattice mismatches between the GaAs and the GaN, an indium gallium phosphate InGaP may be disposed between the GaAs solar cell and the GaN TCC to compensate for the lattice mismatch between the GaN and the GaAs. In order to further compensate for the lattice mismatch between the GaN and InGaP, the interface may be formed as a super lattice or as a graded layer. Alternatively, the interface between the GaN and the InGaP may be formed by the offset method or by wafer fusion. The TCC, in accordance with the present invention, is able to compensate for the lattice mismatches at the interfaces of the TCC while eliminating the need for a cover glass and a relatively expensive germanium substrate.